First Council of Nicaea, Canon XVIII

It has come to the knowledge of the holy and great Synod that, in some districts and cities, the deacons administer the Eucharist to the presbyters, whereas neither canon nor custom permits that they who have no right to offer should give the Body of Christ to them that do offer.  And this also has been made known, that certain deacons now touch the Eucharist even before the bishops.  Let all such practices be utterly done away, and let the deacons remain within their own bounds, knowing that they are the ministers of the bishop and the inferiors of the presbyters.  Let them receive the Eucharist according to their order, after the presbyters, and let either the bishop or the presbyter administer to them.  Furthermore, let not the deacons sit among the presbyters, for that is contrary to canon and order.  And if, after this decree, any one shall refuse to obey, let him be deposed from the diaconate.

Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.